WL#6969: Additional concurrency tests for BGC

RATIONALE
=========
* Concurrency tests verify that there is not deadlock in group commit, non-group
commit and semi sync areas.
SUMMARY
=======
1. Concurrency test verifies that there is not deadlock on group commit when
semi-sync is disabled
2. Concurrency test verifies that there is not deadlock on group commit when
semi-sync is enabled
3. Concurrency test verifies that there is not deadlock on non-group commit when
semi-sync is enabled